Local Area networks (LAN's) are used to facilitate communications between a number of users. Individual LAN's may be bridged together to allow a large number of users to communicate amongst themselves. These bridged LAN's may be further interconnected with other bridged LAN's using routers to form even larger communications networks.
Prior art FIG. 1 depicts an exemplary interconnected bridged LAN system. The numerals 10, 20, 30, etc., are used to identify individual LAN's. Bridges between LAN's are designated by the numerals 5, 15, 25 and 35. A router between bridged LAN 100 and bridged LAN 200 is identified with the reference numeral 300. In the prior art bridged LAN system depicted, a user A is able to communicate with a user B without leaving the LAN 10.
If user A desires to communicate with user C in LAN 20 or user D in LAN 30, the communication is transmitted via bridges 5 and/or 15. If user A desires to communicate with user E, the communication must be routed via router 300 to bridged LAN 200. As will be understood by those skilled in the art, bridges operate at layer 2 of the network model and transparently bridge two LAN's. It is transparent to users A and C that communications between them are ported over bridge 5 because layer 2 bridges do not modify packets, except as necessary to comply with the type of destination LAN. However, if user A wishes to communicate with user E, the communication must be ported via router 300 which operates at level 3 of the network model.
LAN network administrators generally attempt to connect together those users who frequently communicate with each other in bridged LAN's. However, if the bridged LAN becomes too large, it becomes unscalable and may experience various well-known problems. Accordingly, routers are used to interconnect bridged LAN's so that the bridged LAN's themselves can be kept to an acceptable size. This results in delays in communications between users which are transmitted via the router 300. If, for example, in FIG. 1, user E and user A need to communicate frequently, it would be advantageous to interconnect LAN 10 and LAN 50 via a bridge rather than the router 300. This would require system rewiring, which is costly and may be impracticable under many circumstances, such as, if users A and E will only need to frequently communicate for a limited period of time.
It is often beneficial in bridged LAN's and other types of communication systems or networks for redundant communication paths to be provided. Referring again to FIG. 1, a switch 37 in bridged LAN 200 provides a redundant communication path between LAN 50 and LAN 60.
Prior art FIG. 2 depicts another communication system having redundant communications paths. As shown, the system includes LAN's 305-330. LAN 305 is connected to LAN 310 by switch 340. LAN 310 is connected to LAN 315 by a switch 350. This provides a primary communication path between LAN's 305 and 315. Accordingly, during normal operations communications between users X and Y are directed through switches 340 and 350 along the communication path 410. A redundant path 420 is also shown connecting LAN's 305 and 315. This path is under the control of switch 360 which also connects LAN 305 with LAN's 320-330. Conventional switch 360 includes a switch controller which implements forward processing and spanning tree processing, the latter in accordance with a spanning tree protocol.
Each of the switches periodically exchange hello messages, typically at a frequency of once per second. It will be recognized by those skilled in the art that data communications are being received by switch 360 at a substantially higher frequency and that tens of thousands, if not hundreds of thousands of data communications packets may be received by the switch 360 every second. Based upon the spanning tree protocol implemented by the switch 360, data traffic between users X and Y is prohibited by switch 360 from transmission via the redundant communication path 420 as long as the hello messages are periodically received.
If a succession of hello messages are not received from either of switch 340 or switch 350, for example, fifteen successive hello messages are missed, the switch 360, in accordance with the spanning tree protocol, opens the redundant communication path and allows communications between users X and Y to be transmitted via the redundant link 420. This is intended to ensure that the redundant communication path is only available for transmitting communications between LAN's 305 and 315 when the primary communication path 410 has failed. As those skilled in the art will recognize, if both communication paths 410 and 420 are simultaneously open to traffic, a network loop will be formed which will result in an extreme overloading of the system which is, in turn, likely to bring the network down.
Conventional switches 340-360 may have a threshold capacity over which the switch is unable to forward received traffic. Accordingly, each switch is configured such that when the amount of received traffic exceeds the threshold capacity or limit, the excess traffic may be simply dropped. However, this dropping of traffic may also result in anomalies in the switch 360 monitoring of the hello messages. More particularly, if hello messages are dropped along with excess data communications, the switch 360 will erroneously conclude that the primary communication path 410 is inoperable and therefore open the redundant communication path 420 unnecessarily, thereby causing a network loop which will overload and bring down the system.